Conventional optical sights are used in computer controlled plotters to manually optically locate reference points on drawings or the like. Once each reference point is located, signals representative of the located reference point are caused to be sent from the plotter to the computer. The signals representative of the reference point are derived by positioning a carrier mechanism of the plotter relative to a drawing or other material upon which the reference point is located so as to align an optical axis of the sight with the reference point. The optical axis of the sight is at the same relative position on the carrier mechanism as an axis through the marking point of a pen would be if the pen were positioned in the carrier mechanism. For example, optical sights are useful to align drawings for additional or revised marking on them by a pen under the control of the plotter, or to establish a predetermined orientation between the paper or other material to be marked or otherwise operated on and the information in the computer. Successful execution of the computer software information usually requires the use of the optical sight to establish the orientation of the material prior to commencement of the procedure.
To meaningfully use the computer controlled plotter, coincidence between the optical axis of the sight and the axis of the pen is essential, or at least some predetermined relationship is essential. It is therefore usually the situation that both pens and optical sights have essentially the same exterior configuration so the axis through each remains in a single consistent position on the carrier mechanism when each is used. The external dimensions of both the pens and the optical sights are also approximately the same so that the pens and sights can automatically be interchanged on the carrier mechanism by transferring these instruments to a conventional carousel of the plotter and from the carousel to the carrier mechanism under the automatic control of the computer.